Combined fluidized bed and pulverized coal combustion method

ABSTRACT

The invention relates to a combined fluidized bed and pulverized coal combustion method and system. In the method, fluidizing air ( 4 ) is injected into a fluidized bed ( 2 ) situated in the bottom portion of the combustion chamber ( 3 ). Into the combustion chamber, to above the fluidized bed ( 2 ), is fed a mixture of pulverized coal and a carrier gas from a second set of fuel feed means ( 6 ) at a mass flow rate which is higher or at least substantially equal to the upper ignition limit of the mixture, and the mixture of the pulverized coal and the carrier gas at least by the fluidizing air ( 4 ), and at least a fraction of the fuel fed via the second set of fuel feed means ( 6 ) is combusted above the fluidized bed ( 2 ).

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI01/00459 which has an Internationalfiling date of May 11, 2001, which designated the United States ofAmerica.

The invention relates to a combined fluidized bed and pulverized coalcombustion method according to the preamble of claim 1, in which methodfluidizing air is injected into a fluidized bed residing in the bottomportion of a combustion chamber, fuel is fed from a first set of fuelfeed means into the fluidized bed and is burnt in the fluidized bed, andthe mixture of pulverized coal and a carrier gas is fed from a secondset of fuel feed means into the combustion chamber, to above thefluidized bed.

The invention also relates to a system for implementing the method.

In firing by pulverized coal, the pulverized coal and the combustion airare mixed with each other in a burner. To obtain an efficient ignitionand combustion of the mixture of coal and combustion air introduced froma burner into the combustion chamber of a boiler, these two must have asuitable fuel/air ratio. The mixture does not ignite at all if the massflow rate ratio of coal to combustion air is below a certain lowerignition limit. In firing by pulverized coal, the lower ignition limitis typically about 0.2. When the fuel/air ratio is in the range 0.2 to0.4, the mixture will ignite but due to the lean mixture, the flameremains unstable and the combustion temperature low. The generally usedfuel/air ratio in firing by pulverized coal is about 0.4, whereby theflame is stable and the mixture burns at an elevated temperature. Afuel/air ratio higher than this up to a certain upper ignition limitgives good ignition but due to the rich mixture renders a low combustiontemperature. In firing by pulverized coal, the upper ignition limit istypically about 1.0. Mixtures richer than this cannot be ignitedanymore.

In fluidized bed combustion, the fuel burns and becomes partiallygasified in a fluidized bed which resides above an air distributorlocated in the bottom portion of the combustion chamber of the boilerand is formed by particulate matter bed material and the fuel mixedtherewith. Conventionally, the bed material is sand. The bed ismaintained in a fluidized state by way of injecting fluidizing gas,generally air, into the bed from nozzles located in the air distributor.As the velocity of the fluidizing air is low in the bed and a coarseparticle size is selected for the bed material, the fluidized bed isconsequently formed in the bottom portion of the combustion chamber. Thesolid fuel is generally fed into the fluidized bed boiler via fuel feednozzles adapted to the walls of the combustion chamber. The combustiontemperature in fluidized bed combustion is typically about 800 to 950°C.

Due to the low combustion temperature and coarse milling of the fuel,fluidized bed combustion of coal has given a relatively low combustionefficiency as compared with many other firing methods. The lowcombustion temperature also increases the amount of nitrogen oxidesformed in the combustion process. If a coarsely milled fuel of high heatvalue, such as coal, is burnt in a fluidized bed, accumulation ofuncombusted fuel in the bottom portion of the fluidized bed takes place,whereby the fuel burning therein elevates the bed temperature andsintering of the bed material occurs. To avoid this, the bed can becooled by heat exchangers located in the bed. However, the abrasive bedmaterial can rapidly corrode a heat exchanger embedded in the bed. Theamount of fuel accumulating in the bottom portion of the fluidized bedcan be reduced by way of moving the inlet point of the fuel to above thefluidized bed and/or milling the fuel into a smaller particulate size.The latter operation, however, generally dictates the acquisition of acoal mill of a higher milling efficiency.

In U.S. Pat. No. 4,993,332 is described a hybrid combustion system thatcombines fluidized bed combustion with pulverized coal combustion, inwhich system the fluidized bed of a fluidized bed boiler is fired in aconventional manner by coal complemented with firing pulverized coalabove the fluidized bed by means of a burner mounted on the boiler wall.The object of this arrangement is to reduce the disadvantages offluidized bed combustion and pulverized coal combustion.

When a conventional fluidized bed boiler is to be converted suitable forhybrid combustion, the boiler need to be retrofitted with a pulverizedcoal burner and an efficient coal mill in order to provide the boilerwith a coal feed of sufficiently fine and consistent particle size.However, the new burner and efficient coal mill represent a significantcost-increasing factor in retrofitting a hybrid combustion system.

It is an object of the invention to provide an entirely novel fluidizedbed combustion method and system capable of improving the combustionefficiency of fluidized bed combustion and reducing nitrogen oxideemissions of fluidized bed combustion.

In an embodiment according to the invention, the fuel is fed in aconventional manner into the fluidized bed of a fluidized bed boiler andis combusted in the bed. Additionally, to above the fluidized bed isadmitted pulverized coal through, e.g., a duct adapted to the wall ofthe combustion chamber, at such a high coal/carrier gas ratio that thefuel will not ignite in the close vicinity of the feed point. Thefluidizing air blown upward from the bottom of the bed dilutes themixture of the pulverized coal and the carrier gas thus allowing thecoal particles to ignite and burn in a rich flame pattern above thefluidized bed. While a fraction of the coal can burn above the fluidizedbed, the other fraction falls into the fluidized bed and is combustedtherein.

More specifically, the fluidized bed combustion method according to theinvention is characterized by what is stated in the characterizing partof claim 1.

Furthermore, the fluidized bed combustion system according to theinvention is characterized by what is stated in the characterizing partof claim 11.

The invention offers significant benefits.

While the system according to the invention operates generally in thesame fashion as the hybrid combustion system described above, butinstead, the pulverized coal burner of the prior-art system is replacedby a duct exiting into the combustion chamber. Moreover, herein the fuelto be discharged from the duct need not be milled to such a fineness andhomogeneous particle size as in pulverized coal combustion utilizing aburner, thus allowing the system according to the invention to use asimple and cost-efficient coal mill.

A major fraction of the coal fed above the fluidized bed is burnt beforethe coal falls onto the fluidized bed. A certain fraction of the coal iscombusted in the bed, whereby the bed temperature rises slightly thuscontributing to the combustion efficiency of the fluidized bedcombustion system. However, the bed temperature will not rise so high asto necessitate the use of separate heat exchangers for cooling the bed.The coal combusted above the fluidized bed burns under fuel-richconditions at a high temperature, whereby above the fluidized bed iscreated a so-called reburn zone wherein nitrogen oxides formed in thebed are reduced into molecular nitrogen. If a refuse fuel is combustedin the fluidized bed, the combustion process of the bed forms dioxinsand other hazardous compounds that also are destroyed at the elevatedtemperature of the reburn zone. Furthermore, the high temperature of thereburn zone promotes the combustion of coal particles above thefluidized bed, thus improving the combustion efficiency of the boiler.

In the following, the invention is examined in more detail by way ofreferring to the attached drawings, wherein

FIG. 1 is a schematic diagram of an embodiment of a combined fluidizedbed and pulverized coal combustion system according to the invention.

FIG. 2 is a schematic diagram of another embodiment of a combinedfluidized bed and pulverized coal combustion system according to theinvention.

FIG. 3 is a diagram illustrating the amount of nitrogen oxide emissionsand the combustion efficiency as a function of combustion temperature inthe boiler.

A system illustrated in the drawing comprises a fluidized bed boiler 1having a fluidized bed 2 of particulate matter situated in the bottomportion of the boiler combustion chamber 3. A grid 13 disposed at thebottom of the combustion chamber 3 includes air feed means for admittingfluidizing air 4 into the bed material. The velocity of the fluidizingair 4 is adjusted to keep the fluidized bed 2 formed in the bottomportion of the combustion chamber 3 in order to avoid a substantial lossof the bed material from the bed along with the gas flow, thus generallydisposing with the need for circulating the bed material particles backto the bed 2. This type of fluidized bed is also known as a bubblingfluidized bed. Although a fraction of the bed material particles canrise with the gas flow up to the middle portion of the combustionchamber 3, a bubbling fluidized bed 2 has a clearly discernible toplevel. The fluidizing air 4 may also be used as the primary combustionair in the combustion chamber 3. Coarse-milled fuel is fed into thefluidized bed 2 in a conventional manner via a first set of fuel feedmeans 5, such as one or more openings made to the wall of the combustionchamber 3, whereto the fuel is transported by a conveyor from a silo 16.Conventionally, the fuel is coarse-milled coal, peat, biofuel, refusefuel or a mixture of these.

Into the combustion chamber 3 is also fed pulverized coal via a secondset of fuel feed means located above the fluidized bed 2, such as a duct6 exiting into the combustion chamber 3. In its simplest form, the duct6 can be a pipe exiting into the combustion chamber with a diameter of150–300 mm. Also a multiple number of ducts 6 can be used. The coal isground in a coal mill 9 and the coal comminuted into pulverized form istransported pneumatically via a cyclone 10 to a coal storage 11,wherefrom it is moved with the help of a screw conveyor 14, forinstance, to the duct 6. The carrier gas of the pulverized coal ispressurized by means of a compressor 15, thus effecting the pneumaticdischarge of the pulverized coal from the duct 6 into the combustionchamber 3. Advantageously, the carrier gas is air. Alternatively, thecarrier gas may be flue gas from the boiler 1, steam or nitrogen. Thecoal discharged from the duct 6 need not be milled as fine andhomogeneous as is required for the fuel fed to a pulverized coal burnerthus allowing the system according to the invention to operate utilizingan extremely simple type of coal mill 9.

At the entry of the fuel from the duct 6 to the combustion chamber 3,the ratio of the mass flow rate of the pulverized coal to the carriergas is greater or at least substantially equal to the upper ignitionlimit. The ratio of the mass flow rate of the pulverized coal to thecarrier gas is adjusted to a desired value by means of controlling therotational speeds of the screw conveyor 14 and the compressor 15.Advantageously, a 50–60% fraction of the pulverized coal has a particlesize smaller than 74 μm. If the carrier gas is air and the fuel ispulverized coal with a coarse particle size of 70–150 μm, the ratio ofthe mass flow rate of the fuel to the carrier gas air is advantageously1–10, most advantageously 3–7. The velocity of the pulverized coal andthe carrier gas exiting from the duct 6 into the combustion chamber 3 isadvantageously 20–30 m/s, most advantageously about 25 m/s. Secondarycombustion air is admitted into the combustion chamber 3 at the level ofthe duct 6 or thereabove via secondary-air inlet means 7.

The feed point of the mixture of the carrier gas and the pulverized coalis arranged depending on the size of the boiler 1 advantageously at adistance of 1 to 6 m, most advantageously 2 to 4 m, upward from the toplevel of the fluidized bed 2. The feed point can be adapted below thesecondary-air inlet point 7 or at the same level with the secondary airinlet point 7. Typically, the height of the fluidized bed 2 is about 1m, whereby the feed point of the carrier gas and the pulverized coal isabout 2 to 7 m, most advantageously 3 to 5 m above the grid 13. Thefluidizing air 4 dilutes the mixture fed from the duct 6 thus allowingthe particles of the pulverized coal to ignite and burn under fuel-richconditions at a high temperature above the fluidized bed 2. The ratio inthis combustion process is typically 0.5 to 1 kg_(coal)/kg_(air) and thecombustion temperature is 1300 to 1500° C. The temperature is about 200to 300° C. higher than the gas temperature in the freeboard above thefluidized bed of a conventional fluidized bed boiler. Thus, thetemperature and other conditions in the space above the fluidized bed 2are adjusted to an optimal range (cf. FIG. 3), whereby the formation ofnitrogen oxides in combustion is minimized.

Advantageously, at least half, most advantageously 70 to 85%, of thepulverized coal fed via the duct 6 is combusted above the fluidized bed2 and the rest descends into the bed 2 so as to undergo completecombustion in the bed 2. Resultingly, the temperature of the fluidizedbed 2 rises slightly, which improves the combustion efficiency of thefuel fed from the first fuel feed means 5 into the fluidized bed 2.However, the temperature of the bed 2 does not herein rise excessivelyso that the bed material would begin to sinter. If the distance of theduct 6 from the top level of the fluidized bed 2 is made too small, agreater number of coal particles can fall into the fluidized bed 2,whereby the temperature of the fluidized bed 2 begins to rise. Incontrast, if the distance of the duct 6 from the top level of thefluidized bed 2 is made too large, coal particles have enough time toburn down to a too small size before they reach the fluidized bed 2,whereby they are conveyed out from the combustion chamber 3 along withthe flue gases, which is detrimental to the combustion efficiency of theboiler 1.

Due to the high temperature and low oxygen content, above the fluidizedbed 2 is created a so-called reburn zone, wherein the hydrocarbonradicals stemming from the combustion of the fuel convert nitrogenoxides formed in the fluidized bed 2 into hydrogen cyanide.Simultaneously, dioxins and other organic compounds formed in thefluidized bed 2 from combustion of refuse fuel are destroyed.

To above the secondary-air inlet means 7, into the wall of thecombustion chamber 3, are adapted tertiary-air inlet means 8 forinjecting tertiary air into the combustion chamber 3. Herein, anyuncombusted fuel still existing in the flue gas flow is combustedcompletely and hydrogen cyanide molecules formed in the reburn reactionare converted into molecular nitrogen. Fine coal particles, which areseparated in the cyclone 10 from the flow of millded coal beingtransported to the coal storage 1, are fed along with the tertiary airinto the combustion chamber 3, whereby the above-mentioned reburnreaction takes place also at the level of the tertiary air inlet means 8as these fines are combusted. Calcium carbonate or limestone taken froma container 12 is mixed with the tertiary air 8 and the secondary air 7in order to eliminate sulfur compounds from the effluents.

Conventionally a fraction of the total fuel power of the boiler 1 is fedas pulverized coal via the second set of fuel feed means 6, while theother fraction is fed in coarse-milled form via the first set of fuelfeed means 5. The ratio of the fed fuel powers can be varied over a widerange. The first set of fuel feed means 5 and the second set of fuelfeed means 6 may also be used independently from each other. Herein, theentire fuel power of the boiler 1 can be fed via the second set of fuelfeed means 6, while the first set of fuel feed means 5 are closed, andvice versa. However, usually 10–90% of the total fuel power of theboiler 1 is fed to above the fluidised bed 2 via the second set of fuelfeeding means 6.

The boiler 1 may also be operated as shown in FIG. 2, whereby themixture of fuel and carrier gas normally passed to the second set offuel feed means 6 is also discharged in to the combustion chamber 3 vianozzles 17 opening into the fluidized bed 2. Herein, the nozzles 17perform as the first set of fuel feed means. A need to feed fuel via thenozzle 17 into the fluidized bed 2 arises, for instance, when the fuelsupply from the silo 16 is insufficient or the silo 16 is entirelynonoperative. Then, the fuel discharged via the nozzle 17 serves as thepreheating energy source of the fluidized bed 2.

1. Combined fluidized bed and pulverized coal combustion method for usein the combustion chamber (3) of a fluidized bed boiler, the methodcomprising the steps of injecting fluidizing air (4) into a fluidizedbed (2) situated in the bottom portion of the combustion chamber (3),feeding fuel from a first set of fuel feed means (5) into the fluidizedbed (2) and combusting the fuel in the fluidized bed (2), feeding amixture of pulverized coal and a carrier gas from a second set of fuelfeed means (6) into the combustion chamber 3, to above the fluidized bed(2), into the combustion chamber (3), to above the fluidized bed (2) isfed the mixture of the pulverized coal and the carrier gas from thesecond set of fuel feed means (6) at a mass flow rate ratio which ishigher or at least substantially equal to the upper ignition limit ofthe mixture, and the mixture of the pulverized coal and the carrier gasfed from the second set of fuel feed means (6) is diluted at least bythe fluidizing air (4), characterized in that at least a fraction of thefuel fed via the second set of fuel feed means (6) is combusted abovethe fluidized bed (2) at the temperature of 1300 to 1500° C.
 2. Methodaccording to claim 1, characterized in that a fraction of the pulverizedcoal fed via the second set of fuel feed means (6) is combusted in thefluidized bed (2).
 3. Method according to claim 1, characterized in thatpulverized coal with air is fed via the second set of fuel feed means(6) so that the mass flow rate of these is from 1 to 10, mostadvantageously from 3 to
 7. 4. Method according to claim 1,characterized in that secondary air (7) is injected into the combustionchamber (3), to above the second set of fuel feed means (6).
 5. Methodaccording to claim 1, characterized in that tertiary air (8) is injectedinto the combustion chamber (3), to above the level of the secondary-airinlet means (7).
 6. Method according to claim 1, characterized in thatinto the combustion chamber (3) is fed via the second set of fuel feed(6) means the mixture of fuel and carrier gas at a velocity of 20 to 30m/s, most advantageously at a velocity of about 25 m/s.
 7. Methodaccording to claim 1, characterized in that at least half, mostadvantageously 70 to 85%, of the pulverized coal fed via the second setof fuel feed means (6) is combusted above the fluidized bed (2). 8.Method according to claim 1, characterized in that into the combustionchamber (3) is fed via the second set of fuel feed means (6) the mixtureof fuel and carrier gas at a level from 1 to 6 m, most advantageouslyfrom 2 to 4 m, above the top level of the fluidized bed (2).
 9. Methodaccording to claim 1, characterized in that the pulverized coal fed viathe second set of fuel feed means (6) is combusted above the fluidizedbed (2) at a ratio of 0.5 to 1 kg_(coal)/kg_(air).
 10. Combinedfluidized bed and pulverized coal combustion system comprising acombustion chamber (3) with an grid (13) adapted to the bottom portionthereof, a fluidized bed (2) adapted to the bottom portion of thecombustion chamber (3) and formed by a particulate matter bed material,air injection means adapted to the grid (13) for feeding fluidizing air(44) into the bed material, a first set of fuel feed means (5) forfeeding fuel into the fluidized bed (2), the system comprises a secondset of fuel feed means (6) for feeding a mixture of pulverized coal anda carrier gas into the combustion chamber (3), at a distance above thefluidized bed (2) at a mass flow rate ratio which is higher or at leastsubstantially equal to the upper ignition limit of the mixture, and thesecond set of fuel feed means (6) are adapted at a distance above thetop level of the fluidized bed such that the mixture of pulverized coaland a carrier gas discharged from said means becomes dilutable with thefluidizing air (4) and at least in partially combustible above thefluidized bed (2) characterized in that the second set of fuel feedmeans (6) are arranged at such a distance above the top level of thefluidized bed that the combustion temperature of the pulverized coalabove the bed is 1300–1500° C.
 11. System according to claim 10,characterized by means (7) for injecting secondary air into thecombustion chamber (3) at a level above said second set of fuel feedmeans (6).
 12. System for fluidized bed combustion according to claim10, characterized by means (8) for injecting tertiary air into thecombustion chamber (3) at a level above said secondary-air inlet means(7).
 13. System according to claim 10, characterized in that said secondset of fuel feed means comprise at least one duct (6) exiting into thecombustion chamber (3).
 14. System according to claim 10, characterizedin that at least half, most advantageously 70 to 85% of the pulverizedcoal fed via the second set of fuel feed means (6) is combustible abovethe fluidized bed (2).
 15. Method according to claim 2, characterized inthat pulverized coal with air is fed via the second set of fuel feedmeans (6) so that the mass flow rate of these is from 1 to 10, mostadvantageously from 3 to
 7. 16. Method according to claim 4,characterized in that tertiary air (8) is injected into the combustionchamber (3), to above the level of the secondary-air inlet means (7).17. Method according to claim 7, characterized in that the pulverizedcoal fed via the second set of fuel feed means (6) is combusted abovethe fluidized bed (2) at a ratio of 0.5 to kg_(coal)/kg_(air). 18.System for fluidized bed combustion according to aim 11, characterizedby means (8) for injecting tertiary air to the combustion chamber (3) ata level above said secondary-air inlet means (7).